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Photosyn h KEY
CONCEPTS
... Figure 10.1 How can sunlight, seen here as a spectrum of colors in a rainbow, power the synthesis of organic substances"?
10.1 Photosynthesis converts light energy to the
chemical energy of food 10.2 The light reactions convert solar energy to the chemical energy of AlP and NADPH 10.3 The Calvin cycle uses AlP and NADPH to convert CO 2 to sugar 10.4 Alternative mechanisms of carbon fixation have evolved in hot, arid climates
r~:;;;;:ss That Feeds the Biosphere
ife on Earth is solar powered. The chloroplasts of plants capture light energy that has traveled 150 million kilometers from the sun and convert it to chemical energy stored in sugar and other organic molecules. This conversion process is called photosynthesis. Let's begin by placing photosynthesis in its ecological context. Photosynthesis nourishes almost the entire living world directly or indirectly. An organism acquires the organic compounds it uses for energy and carbon skeletons by one of two major modes: autotrophic nutrition or heterotrophic nutrition. Autotrophs are "self-feedersn (auto means "self," and trap/lOs means "feed"); they sustain themselves without eating anything derived from other living beings. Autotrophs produce their organic molecules from CO 2 and other inorganic raw materials obtained from the environment. They are the ultimate sources oforganic compounds for all nonautotrophic organisms, and for this reason, biologists refer to autotrophs as the producers of the biosphere.
L
Almost all plants are autotrophs; the only nutrients they require are water and minerals from the soil and carbon dioxide from the air. Specifically, plants are pholoautotrophs, organisms that use light as a source of energy to synthesize organic substances (Figure 10.1). Photosynthesis also occurs in algae, certain other protists, and some prokaryotes (Figure 10.2, on the next page). In this chapter, we will touch on these other groups in passing, but our emphasis will be on plants. Variations in autotrophic nutrition that occur in prokaryotes and algae will be detailed in Chapters 27 and 28. Hctcrotrophs obtain their organic material by the second major mode of nutrition. Unable to make their own food, they live on compounds produced by other organisms (hetero means "other"). Heterotrophs are the biosphere's consumers. The most obvious form of this "other-feeding" occurs when an animal eats plants or other animals. But heterotrophic nutrition may be more subtle. Some heterotrophs consume the remains of dead organisms by decomposing and feeding on organic litter such as carcasses, feces, and fallen leaves; they are known as decomposers. Most fungi and many types of prokaryotes get their nourishment this way. Almost all heterotrophs, including humans, are completely dependent, either directly or indirectly, on photoautotrophs for food-and also for oxygen, a by-product of photosynthesis. In this chapter, you will learn how photosynthesis works. After a discussion of the general principles of photosynthesis, we will consider the two stages of photosynthesis: the light reactions, in which solar energy is captured and transformed into chemical energy; and the Calvin cycle, in which the chemical energy is used to make organic molecules of food. Finally, we will consider a few aspects of photosynthesis from an evolutionary perspective.
185
.. Figure 10.2 Photoalltotrophs. These organi>ms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living wOOd. Ca) On land, plants are the predominant producers of food. In aquatic environments. photosynthetic organl>ms indude (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria. which produce sulfur (spherical globules) (c, d. e: lMsl路
(a) Plants
(b) Multicellular alga
(d) Cyanobacteria
r;~::;;n~~;~ converts light
Chapter 6. In fact, the original chloroplast is believed to have been a photosynthetic prokaryote that lived inside a eukaryotic cell. (You'll learn more about this hypothesis in Chapter 25.) Chloroplasts are present in a variety of photosynthesizing organisms (see Figure 10.2), but here we will focus on plants.
energy to the chemical energy of food
The remarkable ability of an organism to harness light energy and use it to drive the synthesis oforganic compounds emerges from structural organization in the cell: Photosynthetic enzymes and other mole<ules are grouped together in a biological memo brane, enabling the necessary series of chemical reactions to be carried out efficiently. The process ofphotosynthesis most likely originated in a group of bacteria that had infolded regions of the plasma membrane containing clusters of such molecules. In existing photosynthetic bacteria, infolded photosynthetic membranes function similarly to the internal membranes of the chloroplast, a eukaryotic organelle you learned about in 186
UNIT TWO
The Cell
Chloroplasts: The Sites of Photosynthesis in Plants All green parts of a plant, including green stems and unripened fruit, have chloroplasts, but the leaves are the major sites ofphotosynthesis in most plants (Figure 10.3). There are about half a million chloroplasts per square millimeter of leaf surface. The color of the leaf is from chlorophyll, the green pigment located within chloroplasts. It is the light energy absorbed by chlorophyll that drives the synthesis oforganic molecules in the chloroplast. Chloroplasts are found mainly in the ceUs of the mesophyll, the tissue in the interior of the leaf.
Leaf (fOSS section
Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called stomata (singular, stoma; from the Greek, meaning "mouth"). Water absorbed by the roots is delivered to the leaves in veins. Leaves also use veins to export sugar to roots and other nonphotosynthetic parts of the plant. A typical mesophyll cell has about 30 to 40 chloroplasts, each organelle measuring about 2-4 ~m by 4-7 11m. An envelope of two membranes encloses the stroma, the dense fluid within the chloroplast. An elaborate system of interconnected membranous sacs called thylakoids segregates the stroma from another compartment, the interior of the thylakoids, or thy/akoid space. In some places, thylakoid sacs are stacked in columns called grana (singular, granum). Chlorophyll resides in the thylakoid membranes. (The infolded photosynthetic membranes of prokaryotes are also called thylakoid membranes; see Figure 27.7b.) Now that we have looked at the sites of photosynthesis in plants, we are ready to look more closely at the process of photosynthesis.
vein
Stomata
co,
ChlorOPlast~------t--j~~
Tracking Atoms Through Photosynthesis: Scientific tnquiry Scientists have tried for centuries to piece together the process by which plants make food. Although some of the steps are still not completely understood, the overall photosynthetic equation has been known since the 1800s: In the presence of light, the green parts of plants produce organic compounds and oxygen from carbon dioxide and water. Using molecular formulas, we can summarize the complex series of chemical reactions in photosynthesis with this chemical equation:
6C0 2 + 12 H2 0
Outer membrane
+ Lightenergy-. C6 Hl2 0 6 + 602 + 6 H20
We use glucose (C6 H n 0 6 ) here to simplify the relationship between photosynthesis and respiration, but the direct product of photosynthesis is actually a three-carbon sugar that can be used to make glucose. Water appears on both sides of the equation because 12 molecules are consumed and 6 molecules are newly formed during photosynthesis. We can simplify the equation by indicating only the net consumption of water:
Writing the equation in this form, we can see that the overall chemical change during photosynthesis is the reverse of the one that occurs during cellular respiration. Both of these metabolic processes occur in plant cells. However, as you will soon learn, chloroplasts do not synthesize sugars by simply reversing the steps of respiration.
Stroma Granum
Thylakoid space
Intermembrane space Inner membrane
... Figure 10.3 Zooming in on the location of photosynthesis in a plant. Leaves are the major organs of photosynthesis in plants. These pictures take you into a leal. then into a cell, and finally into a chloroplast. the organelle where photosynthesis occurs (middle. LM; bottom, TEM),
CHAPTH TEN
Photosynthesis
187
Now let's divide the photosynthetic equation by 6 to put it in its simplest possible form:
Here, the brackets indicate that CH 2 0 is not an actual sugar but represents the general formula for a carbohydrate. In other words, we are imagining the synthesis of a sugar mol· ecule one carbon at a time. Six repetitions would theoretically produce a glucose molecule. Let's now use this simplified for· mula to see how researchers tracked the elements Co H, and o from the reactants of photosynthesis to the products.
The Splitting of Water One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given offby plants is derived from H2 0 and not from CO 2, The chloroplast splits water into hydrogen and oxygen. Before this discovery, the prevailing hypothesis was that photosynthesissplitcarbondioxide{C02 ---+ C +00 and then added water to the carbon (C + H20 ---+ [CH20]). This hypothesis predicted that the O2 released during photosynthesis came from CO 2, This idea was challenged in the 1930s by C. B. van Niel, of Stanford University. Van Niel was investigating pho-tosynthesis in bacteria that make their carbohydrate from CO 2 but do not release O2 , Van Niel concluded that, at least in these bacteria, CO2 is not split into carbon and oxygen. One group of bacteria used hydrogen sulfide (H 2S) rather than water for pho· tosynthesis, forming yellow globules of sulfur as a waste prod· uct (these globules are visible in Figure 1O.2e). Here is the chemical equation for photosynthesis in these sulfur bacteria:
Van Niel reasoned that the bacteria split H2 S and used the hydrogen atoms to make sugar. He then generalized that idea, proposing that all photosynthetic organisms require a hydrogen source but that the source varies: Sulfur bacteria: CO2 Plants: CO2 General: CO2
+ 2 H2S -. [CH20J + H20 + 2 S
+ 2 H20 ---+ [CH2 0j + H20 + O2 + 2 H2 X ---+ [CH2 0j + H20 + 2 X
Thus, van Niel hypothesized that plants split H20 as a source of electrons from hydrogen atoms, releasing O2 as a by-product. Nearly 20 years later, scientists confirmed van Niel's hypothesis by using oxygen-18 ('80), a heavy isotope, as a tracer to follow the fate of oxygen atoms during photosynthesis. The experiments showed that the O 2 from plants was labeled with 180 only if water was the source of the tracer (experiment I). If the ISO was introduced to the plant in the form of CO 2 , the label did not turn up in the released O 2 (experiment 2). In the following summary, red denotes labeled atoms of oxygen esO): Experiment 1: CO 2 + 2 Hp ---+ [CHPJ Experiment 2: CO 2 + 2 H20 ---+ [CH 20J 188
UNIT TWO
The Cell
+ H2 0 + O2 + H2 0 + O2
Reactants:
Products:
... Figure 10.4 Tracking atoms through photosynthesis. The atoms from COl are shown in orange. and the atoms from HlO are shown in blue.
A significant result of the shuffling of atoms during photosynthesis is the extraction of hydrogen from water and its incorporation into sugar. The waste product of photosynthesis, 0 21 is released to the atmosphere. Figure 10.4 shows the fates of all atoms in photosynthesis.
Photosynthesis as a Redox Process Let's briefly compare photosynthesis with cellular respiration. Both processes involve redox reactions. During cellular respiration, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming water as a by-product. The electrons lose potential energy as they "fall" down the electron transport chain toward electronegative oxygen, and the mitochondrion harnesses that energy to synthesize ATP (see Figure 9.16). Photosynthesis reverses the direction of electron flow. Water is split, and electrons are transferred along with hydrogen ions from the water to carbon dioxide, reducing it to sugar. Because the electrons increase in potential energy as they move from water to sugar, this process requires energy, in other words is endergonic. This energy boost is provided by light.
The Two Stages of Photosynthesis: A Preview The equation for photosynthesis is a deceptively simple summary of a very complex process. Actually, photosynthesis is not a single process, but two processes, each with multiple steps. These two stages of photosynthesis are known as the light reactions (the plwto part of photosynthesis) and the Calvin cycle (the synthesis part) (Figure 10.5). The light reactions are the steps ofphotosynthesis that convert solar energy to chemical energy. Water is split, providing a source ofelectrons and protons (hydrogen ions, H+) and giving offO 2 as a by·product Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored. TIle electron acceptor NADP+ is first cousin to NAD+, which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence ofan extra phosphate group in the NADP+ molecule. The light reactions use solar power to reduce NADP+ to NADPH by adding a
.. Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast. the thylakoid membranes are the sites of the light reactions, whereas the Calvin cycle occurs in the stroma, The light reactions use solar energy to make ATP and NADPH, which supply chemICal energy and reducing power, respectively, to the Calvin cycle, The Calvin cycle incorporates (0 2 into organic molecules, which are converted to sugar, (Recall that most simple sugars have formulas that are some multiple of (H 20.)
•
A smaller version of this diagram will reappear in several subsequent figures as a reminder of whether the events being described occur in the light reactions or in the (alvin cycle.
light
Reactions
JJioffix Visit the Study Area at www.masteringbio.com for the BioFlix 3·D Animation on Photosynthesis.
~$,;'
INADPH I~
Chloroplast
pair of electrons along with an H+. The light reactions also generate ATp, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of tv.'o compounds: NADPH, a source of electrons as "reducing power~ that can be passed along to an electron acceptor, reducing it, and ATP, the versatile energy currency ofceUs. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle. The Calvin cycle is named for Melvin Calvin, who, along with his colleagues, began to elucidate its steps in the late 194Os. The cycle begins by incorporating CO 2 from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired its cargo of electrons in the light reactions. To convert CO 2 to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions. Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the NADPH and ATP produced by the light reactions. The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions, or Jightindependent reactions, because none of the steps requires light directly. Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the light reactions
provide the NADPH and ATP that the Calvin cycle requires. In essence, the chloroplast uses light energy to make sugar by coordinating the {v,'O stages of photosynthesis. As Figure 10.5 indicates, the thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the stroma. In the thylakoids, molecules of NADP+ and ADP pick up electrons and phosphate, respectively, and NADPH and ATP are then released to the stroma, where they play crucial roles in the Calvin cycle. The two stages of photosynthesis are treated in this figure as metabolic modules that take in ingredients and crank out products. Our next step toward understanding photosynthesis is to look more closely at how the h·...o stages work, beginning with the light reactions. CONCEPT
CHECK
10.1
I. How do the reactant molecules of photosynthesis reach the chloroplasts in leaves? 2. How did the use of an oxygen isotope help elucidate the chemistry of photosynthesis? 3. •i,'l:f.iIl4 The Calvin cycle clearly requires the products ofthe light reactions, ATP and NADPH. Suppose a classmate asserts that the converse is not truethat the light reactions don't depend on the Calvin cycle and, with continual light, could just keep on producing ATP and NADPH. Do you agree or disagree? Explain. For suggested answers, see Appendix A.
CHAPTH TEN
Photosynthesis
189
r;~:~~;:. :~~~ions convert solar
Chloroplasts are chemical factories powered by the sun. Their thylakoids transform light energy into the chemical energy of ATP and NADPH. To understand this conversion better, we need to know about some important properties oflighl
versely related to the wavelength of the light: the shorter the wavelength, the greater the energy ofeach photon ofthat light. Thus, a photon of violet light packs nearly twice as much energy as a photon of red light. Although the sun radiates the full spectrum ofelectromagnetic energy, the atmosphere acts like a selective window, allowing visible light to pass through while screening out a substantial fraction of other radiation. The part of the spectrum we can see-visible light-is also the radiation that drives photosynthesis.
The Nature of Sunlight
Photosynthetic Pigments: The light Receptors
Ught is a form of energy known as electromagnetic energy, also called electromagnetic radiation. Electromagnetic energy travels in rhythmic waves analogous to those created by dropping a pebble into a pond. Electromagnetic "''aves, however, are disturbances of electric and magnetic fields rather than disturbances of a material medium such as water. The distance between the crests of electromagnetic waves is called the wavelength. Wavelengths range from less than a nanometer (for gamma rays) to more than a kilometer (for ra-
When light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are known as pigments. Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear. If a pigment is illuminated with white light, the color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it appears black.) We see green when .....e look at a leaf because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light (Figure 10.7). The ability of a pigment to absorb various wavelengths of light can be measured with an instrument called a spectrophotometer. This machine directs beams of light of different wavelengths through a solution of the pigment and measures the fraction of the light transmitted at each wavelength. Agraph plotting
energy to the chemical energy of AlP and NADPH
dio waves). This entire range of radiation is known as the electromagnetic spectrum (Figure 10.61. The segment most
important to life is the narrow band from about 380 nm to 750 nm in wavelength. This radiation is known as visible light
because it can be detected as various colors by the human eye. The model of light as waves explains many of light's properties, but in certain respects light beha\'es as though it consists of discrete particles, called photons. Photons are not tangible objects, but they act like objects in that each of them has a fixed quantity of energy. The amount of energy is in-
light Refl~cte/
h 1;9 /
Transmitted light .6. Figure 10.6 The electTOmagnetic spectrum. Vllhrte light IS a mixture of all w~ngthi of visible light. A pnsm un sort white ight Into ItS component colors by bending light of diffl!feOt wavek>ngths at diffl!feOt angles. (Droplets of water In the atmosphere un act as pnsms, foonmg a rainbow; see figure 10.1.) Visible light drIVeS photosynthesis.
190
UNIT TWO
TheCeU
.6. Figure 10.7 Why leaves are green: interaction of light with chloroplasts. The chlorophyll molecules of chloroplasts absorb VIoIet路blue and red light (the colors most effective In driving photosynthesrs) and reflect or transmit grC@fl light. This is why leaves appear green.
a pigment's light absorption versus wavelength is called an absorption spectrum (figure 10.8). The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis, since light can perform work in chloroplasts only ifit is absorbed. figure 10.9a shows the absorption spectra ofthree types ofpigments in chloroplasts: chlorophyll a, which participates directly in the light reactions; the accessory pigment chlorophyll hi and a group ofaccessory pigments called carotenoids. The spectrum ofchlorophyll a suggests that
...
.. Fl~'0.9
In ui
Which wavelengths of light are most effective in driving photosynthesis? EXPERIMENT
Absorption and action spectra, along with a classic experiment by Theodor W. Engelmann, reveal which wave· lengths of light are photosynthetically important.
RESULTS Chlorophyll a
Chlorophyll b
••
10.8 ••
Carotenoids
Determining an Absorption Spectrum
APPLICATION An absorption spectrum is a visual representa· tion of how well a particular pigment absorbs different wavelengths of visible light. Absorption spectra of various chloroplast pigments help scientists decipher each pigment's role in a plant. TECHNIQUE A spectrophotometer measures the relative amounts of light of different wavelengths absorbed and transmit· ted by a pigment solution.
00
600
500
Wavelength of light (nm) (al Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.
o White light is separated into colors (wavelengths) by a prism. f) One by one, the different colors of light are passed through the sample (chlorophyll in this example), Green light and blue light are shown here,
(} The transmitted light strikes a photoelectric tube, which converts the light energy to electricity.
o The electncal current is measured by a galvanometer. The meter indicates the fraction of light transmitted through the sample, from which we can determine the amount of light absorbed,
White light
Refracting prism
Chlorophyll Photoelectric solution tube
(bl Action spectrum. This graph plots the rate of photosynthesis versus wavelength, The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a), This is partly due to the absorption of light by accessory pigments such as chlorophyll band carotenoids,
Galvanometer
Slit moves to pass light of selected wavelength
Green light
Blue light
The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.
The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light.
RESULTS See Figure 10,9a for absorption spectra of three types of chloroplast pigments.
(c) Engelmann's experiment. In 1883, Theodor W. Engelmann
illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths, He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. CONCLUSION light in the violet-blue and red portions of the spectrum is most effective in driving photosynthesis. SOURCE
T, W EngellT\ilnn,
8.Ktenum photomelfl(um
Ein Betr~g
Z\Jr II€rgleochenden Phy\.lolog,e des Li<ht- und fart!En)U1nes, Archiv. fOr PIlysioJogie, 30:95-124 (1883),
.','@il l •
If Engelmann had placed a red·colored filter between the prism and the alga, how would the results have differed?
CHAPTH TEN
Photosynthesis
191
violet-blue and red light work best for photosynthesis, since they are absorbed, while green is the least effective color. This is confirmed by an action spectrum for photosynthesis (Figure 10.9b), which profiles the relative effectiveness of dif~ ferentwavelengthsof radiation in driving the process. An action spectrum is prepared by illuminating chloroplasts with light of different colors and then plotting \\'avelength against some measure of photosynthetic rate, such as CO 2 consumption or Ch release. The action spectrum for photosynthesis was first demonstrated by a German botanist in 1&83. Before equipment for measuring O2 levels had even been invented, 111eodor W. Engelmann performed a clever experiment in which he used bacteria to measure rates of photosynthesis in filamentous algae (Figure 10.9c). His results are a striking match to the modern action spectrum shown in Figure 1O.9b. Notice by comparing Figures 1O.9a and 1O.9b that the ac~ tion spectrum for photosynthesis does not exactly match the absorption spectrum of chlorophyll a. The absorption spectrum of chlorophyll a alone underestimates the effectiveness ofcertain wavelengths in driving photosynthesis. This is partly because accessory pigments with different absorption spectra are also photosynthetically important in chloroplasts and broaden the spectrum of colors that can be used for photosynthesis. Figure 10.10 shows chlorophyll a compared to one
CH,
ClIO
in chlorophyll a in chlorophyll b
Porphyrin ring: light-absorbing "head" of molecule; note magnesium atom at center
Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; Hatoms not shown
.... Figure 10.10 Structure of chlorophyll molecules in chloroplasts of plants. Chlorophyll a and chlorophyll b differ only in one of the functional groups bonded to the organic structure called a porphyrin ring. 192
UNlllWO
The Cell
ofthese accessory pigments, chlorophyll b. Aslight structural difference between them is enough to cause the two pigments to absorb at slightly different wavelengths in the red and blue parts of the spectrum (see Figure 1O.9a). As a result, chloro~ phyll a is blue green and chlorophyll b is olive green. Other accessory pigments include carotcnoids, hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-green light (see Figure to.9a). Carotenoids may broaden the spectrum of colors that can drive photosynthesis. However, a more important function of at least some carotenoids seems to be photoprotection: These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen, forming reactive oxidative molecules that are dangerous to the cell. Interestingly, carotenoids similar to the photoprotective ones in chloroplasts have a photoprotective role in the human eye. These and related molecules, often found in health food products, are valued as "phytochemicals" (from the Greek phyton, plant), compounds with antioxidant properties. Plants can synthesize all the antioxidants they require, but humans and other animals must obtain some of them from their diets.
Excitation of Chlorophyll by Light What exactly happens when chlorophyll and other pigments absorb light? The colors corresponding to the absorbed wavelengths disappear from the spectrum of the transmitted and reflected light, but energy cannot disappear. When a molecule absorbs a photon of light, one of the molecule's electrons is elevated to an orbital where it has more potential energy. \'(fhen the electron is in its normal orbital, the pigment molecule is said to be in its ground state. Absorption ofa photon boosts an electron to an orbital of higher energy, and the pigment molecule is then said to be in an excited state. The only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and an excited state, and this energy difference varies from one kind of molecule to another. Thus, a particular compound absorbs only photons corresponding to specific wavelengths, which is why each pigment has a unique absorption spectrum. Once absorption of a photon raises an electron from the ground state to an excited state, the electron cannot remain there long. The excited state, like all high-energy states, is unstable. Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground-state orbital in a billionth ofa second, releasing tlleir excess energy as heat. This conversion oflight energy to heat is what makes the top ofan automobile so hot on a sunny day. (White cars are coolest because their paint reflects all wavelengths of visible light, although it may absorb ultraviolet and other invisible radiation.) In isolation, some pigments, including chlorophyll, emit light as well as heat after absorbing photons. As excited electrons fall back to the ground state, photons are given off. This afterglow is called
<II Figure 10.11 Excitation of isolated chlorophyll by light. (a) Absorption of a photon causes a transition of the chlorophyll molecule from its ground state to its excited state. The photon boost an electron to an orbital where it has more potential energy. If the illuminated molecule exists in isolation, its excited electron immediately drops back down to the ground-state orbital, and its excess energy is given off as heat and fluorescence (Iightl. (b) A chlorophyll solution excited with ultraviolet light fluoresces with a red-orange glow. If a leaf containing a similar concentration of chlorophyll as the solution was exposed 10 the same ultraviolet light, no fluorescence would be seen. Explain the difference in fluorescence emission between the solution and the leaf.
,-
-'m,nIM Ground
state
(al Excitation of isolated chlorophyll molecule
(b) Fluorescence
fluorescence. If a solution of chlorophyll isolated from chloroplasts is illuminated, it\\ill fluoresce in the red-orange part ofthe spectrum and also give off heat (figure 10.11).
Thylakoid
APhotosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes Chlorophyll molecules excited by the absorption of light energy produce very different results in an intact chloroplast than they do in isolation (see Figure 10.11). In their native environment of the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into photosystems. A photosystcm is composed of a protein complex called a reaction-center complex surrounded by severa/light-harvesting complexes (Figure 10.12). The reaction-center complex includes a special pair of chlorophyll a molecules. Each lightharvesting complex consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and carotenoids) bound to proteins. The number and variety of pigment molecules enable a photosystem to harvest light over a larger surface and a larger portion of the spectrum than any single pigment molecule alone could. Together, these lightharvesting complexes act as an antenna for the reactioncenter complex. When a pigment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a light-harvesting complex, somewhat like a human "wave at a sports arena, until it is passed into the reactioncenter complex. The reaction-center complex contains a molecule capable of accepting electrons and becoming reduced; it is called the primary electron acceptor. The pair of chlorophyll a molecules in the reaction-center complex are special because their molecular environment-their location and the other molecules with which they are associated-enables H
Photon
Photosystem
STROMA
~~~A'----~---,
Primary electron acceptor
Transfer of energy
Special pair of chlorophyll a molecules
Pigment molecules
THYLAKOID SPACE (INTERIOR OF THYLAKOID)
... Figure 10.12 Howa photosystem harvests light. When a photon strikes a pigment molecule in a light-harvesting complex, the energy is passed from moleClJle to moleClJle until it reaches the reactioncenter complex. Here, an excited electron from the special pair of chlorophyll a molecules is transferred to the primary electron acceptor,
them to use the energy from light not only to boost one oftheir electrons to a higher energy level, but also to transfer it to a different molecule-the primary electron acceptor. CHAPTH TEN
Photosynthesis
193
The solar-powered transfer ofan electron from the reactioncenter chlorophyll a pair to the primary electron acceptor is the first step of the light reactions. As soon as the chlorophyU electron is excited to a higher energy level, the primary electron acceptor captures it; this is a redox reaction. Isolated ch1orophyU fluoresces because there is no electron acceptor, so electrons of photocxcitedchlorophyU drop right back to the ground state. In a chloroplast, the potential energy represented by the excited electron is not lost Thus, each photosystem-a reaction-center complex surrounded by light-han'eSling complexes-functions in the chloroplast as a unit It converts light energy to chemical energy, which ....ill ultimately be used for the synthesis ofsugar. The thytakoid membrane is populated by two types of pho路 tosystems that cooperate in the light reactions of photosynthe路 sis. They are called photosystem II (PS II) and photosystem I (PS I). (They were named in order of their discovery, but photosystem II functions first in the light reactions.) Each has a characteristic reaction-center complex-a particular kind of primary electron acceptor next to a special pair ofchlorophyll a molecules associated with specific proteins. The reactioncenter chlorophyll a ofphotosystem Uis known as P680 because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum). The chlorophyll a at the reaction-<enter complex: of photosystem Iis called 1'700 because it most effectively absorbs light ofwavelength 700 nm (in the far-red part of the spectrum). These two pigments, P680 and 1'700, are nearly identical chlorophyll a molecules. How路 ever, their association with different proteins in the thylakoid membrane affects the electron distribution in the two pigments and accounts for the slight differences in their light-absorbing properties. Now let's see how the two photosystems work together in using light energy to generate ATP and NADPH, the two main products ofthe light reactions.
8
Linear Electron Flow
o
Lightdrives the synthesis ofATP and NADPH by energizing the two photosystems embedded in the thylakoid membranes of chloroplasts. The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane. This is called Linear electron now, and it occurs during the light reactions of photosynthesis, as shown in Figure 10.13. The numbers in the text description correspond to the numbered steps in the figure.
o
o
194
A photon of light strikes a pigment molecule in a Iightharvesting complex, boosting one of its electrons to a higher energy level. As. this electron falls back to its ground state, an electron in a nearby pigment molecule issimultaneously raised to an excited state. TIle process continues, with the energy being relayed to other pigment molecules until it reaches the P680 pair ofchlorophyU a molecules in the PS II reaction-center complex. It excites an electron in this pair of chlorophylls to a higher energy state. UNIT TWO
TheCell
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"
o
This electron is transferred from the excited P680 to the primary electron acceptor. We can refer to the resulting form of P680, missing an electron, as P680+. An enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ions, and an oxygen atom The electrons are supplied one by one to the P680+ pair, each electron replacing one transferred to the primary electron acceptor. (P680+ is the strongest biological oxidizing agent known; its electron ~hole~ must be filled. This greatly facilitates the transfer ofelectrons from the split water molecule.) The oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming 01' Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain, the components of which are similar to those of the electron transport chain that functions in cellular respiration. The electron transpoTt chain between P$ II and PS I is made up ofthe electron carrier plastoquinone (Pq), acytochrome complex, and a protein called plastocyanin (Pc). The exergonic ~fall~ of electrons to a lower energy level provides energy for the synthesis of ATP. As. electrons pass through the cytochrome complex, the pumping of protons builds a proton gradient that is subsequently used in chemiosmosis. Meanwhile, light energy was transferred via lightharvesting complex pigments to the PS I reaction-center complex, exciting an dectron of the P700 pair of chlorophyU a molecules located there. The photoexcited electron was then transferred to PS I's primary electron acceptor, creating an electron "hole" in the P7oo-which we now can call P700+. In other words, P700+ can now act as an electron acceptor, accepting an electron that reaches the bottom ofthe electron transport chain from PS II. Photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor ofPS Idown a second electron transport chain through the protein ferredoxin (Fd). (This chain does not create a proton gradient and thus does not produce ATP.) The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+. Two electrons are required for its reduction to NADPH. This molecule is at a higher energy level than water, and its electrons are more readily available for the reactions of the Calvin cycle than were those ofwater.
As complicated as the scheme shown in Figure 10.13 is, do not lose track of its functions. The light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, respectively, to the carbohydratesynthesizing reactions ofthe Calvin cycle. The energy changes ofelectrons as they flow through the light reactions are shown in a mechanical analogy in Figure 10.14.
... Figure 10.13 How linear electron flow during the light reactions generates ATP and NAOPH. The gold arrows trace the current of light路driven electrons from water to NADPH.
NADf>+ reductase
o light
NADPH
light
Photosystem I Photosystem
n
(PS
n
(PS II)
Cyclic Electron Flow
.. Figure 10.14 A mechanical analogy for the light reactions.
In certain cases, photoexcited electrons can take an alternative path called cyclic electron flow, which uses photosystem I but not photosystem II. You can see in Figure 10.15, on the next page, that cyclic flow is a short circuit: The electrons cycle back from ferredoxin (Fd) to the cytochrome complex and from there continue on to a P700 chlorophyll in the PS I reactioncenter complex. There is no production ofNADPH and no release of oxygen. Cyclic flow does, however, generate ATP. Several of the currently existing groups of photosynthetic bacteria are known to have photosystem I but not photosystem II; for these species, which include the purple sulfur bacteria (see Figure IO.2e), cyclic electron flow is the sole means of generating ATP in photosynthesis. Evolutionary biologists believe that these bacterial groups are descendants of the bacteria in which photosynthesis first evolved, in a form similar to cyclic electron flow. Cyclic electron flow can also occur in photosynthetic species that possess both photosystems; this includes some prokaryotes, such as the cyanobacteria shown in Figure lO.2d, as well as the eukaryotic photosynthetic species that have been tested to CHAPTH TEN
Photosynthesis
195
,. Figure 10.15 Cyclic electron flow. PhotO€xcited electrons from PS I are occasionally shunted back from ferredoxin (Fd) to chlorophyll via the cytochrome complex and plastocyanin (Pc). This electron shunt supplements the supply of ATP (via chemiosmosis) but produces no NADPH, The "shadow" of linear electron flow is included In the diagram for comparison with the cyclic route. The two ferredoxin molecules shown in this diagram are actually one and the samethe final electron carrier in the electron transport chain of PS I.
Primary acceptor
Fd
NADPH
'.' • •
... . •••• ~
Photosystem J
Photosystem II
date. Although the process is probably in part an "evolutionary leftover,~ it clearly plays at least one beneficial role for these organisms. Mutant plants that are not able to carry out cyclic electron flow are capable of growing well in low light, but do not grow well where light is intense. This is evidence for the idea that cyclic electron flow may be photoprotective, protecting cells from light-induced damage. Later youll learn more about cyclic electron flow as it relates to a particular adaptation of photosynthesis (C4 plants; see Concept 10.4). \'V'hether ATP synthesis is driven by linear or cyclic elec· tron flow, the actual mechanism is the same. Before we move on to consider the Calvin cycle, let's review chemiosmosis, the process that uses membranes to couple redox reactions to ATP production.
photosystems capture light energy and use it to drive the electrons from water to the top of the transport chain. In other words, mitochondria use chemiosmosis to transfer chemical energy from food molecules to ATP, whereas chloroplasts transform light energy into chemical energy in ATP. Although the spatial organization of chemiosmosis differs slightly between chloroplasts and mitochondria, it is easy to see similarities in the two (Figure 10.16). The inner membrane of
[ KiJ •
HigherlWj LowerlW] Mitochondrion
Chloroplast
AComparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts and mitochondria generate ATP by the same basic mechanism: chemiosmosis. An electron transport chain assembled in a membrane pumps protons across the membrane as electrons are passed through a series of carriers that are progressively more electronegative. In this way, electron transport chains transform redox energy to a proton-motive force, potential energy stored in the form of an H+ gradient across a membrane. Built into the same membrane is an ATP synthase complex that couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP. Some of the electron carriers, including the iron-containing proteins called cytochromes, are very similar in chloroplasts and mitochondria. The ATP synthase complexes of the two organelles are also very much alike. But there are noteworthy differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. In mitochondria, the highenergy electrons dropped down the transport chain are extracted from organic molecules (which are thus oxidized), while in chloroplasts, the source of electrons is water. Chloroplasts do not need molecules from food to make ATP; their
1%
UNlllWO
The Cell
CHLOROPLAST STRUCTURE
Inner membrane ATP
Matrix
synthase \_"".1 • ADP+®,
Stroma
..;..~ w~
J. Figure 10.16 Comparison of chemiosmosis in mitochondria and chloroplasts. In both kinds of organelles, electron transport chains pump protons (H+) across a membrane from a region of low W concentration (light gray in this diagram) to one of high W concentration (dark gray), The protons then diffuse back across the membrane through ATP synthase, driving the synthesis of ATP.
driving ATP synthesis. In the chloroplast, ATP is synthesized as the hydrogen ions diffuse from the thylakoid space back to the stroma through ATP synthase complexes, whose catalytic knobs are on the stroma side of the membrane. Thus, ATP forms in the stroma, where it is used to help drive sugar synthesis during the Calvin cycle (Figure 10.17). The proton (H+) gradient, or pH gradient, across the thylakoid membrane is substantial. \'(fhen chloroplasts in an experimental setting are illuminated, the pH in the thylakoid space drops to about 5 (the H+ concentration increases), and the pH in the stromaincreases to about 8 (the H+ concentration decreases). This gradient of three pH units corresponds to a thousandfold
the mitochondrion pumps protons from the mitochondrial matrix out to the intermembrane space, which then serves as a reservoir of hydrogen ions. The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space (interior of the thylakoid), which functions as the H+ reservoir. If you imagine the cristae of mitochondria pinching off from the inner membrane, this may help you see how the thylakoid space and the intermembrane space are comparable spaces in the two organelles, while the mitochondrial matrix is analogous to the stroma of the chloroplast. In the mitochondrion, protons diffuse down their concentration gradient from the intermembrane space through ATP synthase to the matrix,
[CH 20] (sugar)
STROMA (low W concentration) Photosystem 11
THYLAKOIO SPACE (high W concentration)
Cytochrome complex
Photosystem I
IhO +2W
4W
•.'I'I'I'I'~·. • 1.1'1.1.1 1.1.1.,.
~~~~~.~: Thylakoid membrane STROMA (low W concentration)
... Figure 10.17 The light reactions and chemiosmosis: the organization of the thylakoid membrane. This diagram shows a ClJrrent model for the organization 01 the thylakoid membrane. The gold arrows track the linear electron flow outlined in Figure 10 13 A5 electrons pass from carrier to carrier in redox reactions. hydrogen ions remo~ed from the stroma are deposited in the thylakoid space,
To Cal~in
Cycle
--i::-II-J
ATP synthase
•
+H'
storing energy as a proton-motive lorce (H' gradient). At least three steps in the light reactions contribute to the proton gradient: 0 Water is split by photosystem II on the side of the membrane facing tile thylakoid space; 8 as plastoquinone (Pq), a mobile carrier, transfers electrons to the cytochrome complex, lour protons are translocated across the membrane into the thylakoid space; and a hydrogen ion is
e
removed Irom the stroma when it is taken up by NAOP' Notice how, asin Figure 10,16, hydrogen ions are being pumped Irom the stroma into the thylakoid space. The diffusion of H+ from the thylakoid space back to the stroma (along the W concentration gradient) pcM'ers the ATP synthase, These light-dri~en reactions store chemical energy in NAOPH and Alp, which shuttle the energy to the carbohydrate-producing Calvin cycle,
CHAPTH TEN
Photosynthesis
197
difference in H+ concentration. If in the laboratory the lights are turned off, the pH gradient is abolished, but it can quickly be restored by rurning the lights back on. Experiments such as this provided strong evidence in support ofthe chemiosmotic model Based on studies in several laboratories. Figure 10.17 shows a current model for the organization ofthe light-reaction ~ma chinery" within the thylakoid membrane. Each of the molecules and molecular complexes in the figure is present in numerous copies in each thylakoid. Notice that 'ADPH, like AT?, is produced on the side of the membrane facing the stroma, where the Calvin cycle reactions take place. Let's summarize the light reactions. Electron flow pushes electrons from water, where they are at a low state of potential energy, ultimately to NADPH, where they are stored at a high state of potential energy. The light-driven electron current also generates ATP. Thus, the equipment of the thylakoid membrane COll\'erts light energy to chemical energy stored in ATP and NADPH. (Oxygen is a by-product.) Let's now see how the Calvin cycle uses the products of the light reactions to synthesize sugar from CO2 ,
10.Z I. \Vhat color of light is least effective in driving photo-
synthesis? Explain. 2. Compared to a solution of isolated chlorophyU. why do intact chloroplasts release less heat and fluorescence when illuminated? 3. In the light reactions, what is the initial electron donor? \Vhere do the electrons end up? 4. •',11:".14 In an experiment, isolated chloroplasts placed in a solution with the appropriate components can carry out ATP synthesis. Predict what would happen to the rate of synthesis if a compound is added to the solution that makes membranes freely permeable to hydrogen ions. For suggested answers. see AppendiX A.
r;~:~:~i~~~le
uses AlP and NADPH to convert CO 2 to sugar
The Calvin cycle is similar to the citric acid cycle in that a starting material is regenerated after molecules enter and leave the cycle. Howe\'er. while the citric add cycle iscatabolic, oxidizing glucose and using the energy to synthesize AT?, the Calvin cycle is anabolic, building carbohydrates from smaller molecules and consuming energy. Carbon enters the Calvin cycle in the form of CO 2 and 1ea\'eS in the form of sugar. The cycle spends ATP as an energy source and consumes NADPH as reducing po.....er for adding high-energy electrons to make the sugar. 198
UNiT
rwo TheCeU
As we mentioned previously, the carbohydrate produced directly from the Calvin cycle is actually not glucose, but a three-carbon sugar; the name ofthis sugar is glyceraldehyde3-phosphate (G3P). For the net synthesis of one molecule of G3P, the cycle must take place three times, fixing three molecules of CO 2 , (Recall that carbon fixation refers to the initial incorporation of CO 2 into organic material.) As we trace the steps of the cycle, keep in mind that we are following three molecules of CO 2 through the reactions. Figure 10.18 divides the Calvin cycle into three phases: carbon fixation, reduction, and regeneration of the CO 2 acceptor. Phase I: Carbon fixation. The Calvin cycle incorporates each CO 2 molecule, one at a time, by attaching it to a fivecarbon sugar named ribulose bisphosphate (abbreviated RuBP). The enzyme that catalyzes this first step is RuBP carboxylase, or rubisco. (This is the most abundant protein in chloroplasts and is also said to be the most abundant protein on Earth.) The product of the reaction is a six·carbon intermediate so unstable that it immediately splits in half, forming t\'o·o molecules of3·phosphoglycerate (for each CO 2 fixed). Phase 2: Reduction. Each moleculeof3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1,3·bisphosphoglycerate. Next, a pair ofelectrons donated from NADPH reduces 1,3-bisphosphoglycerate, which also loses a phosphate group, becoming G3P. Specifically, the electrons from NADPH reduce a carboyxl group on 1,3-bisphosphoglycerate to the aldehyde group of G3P, which stores more potential energy. G3P is a sugar-the same three-carbon sugar formed in glycolysis by the splitting of glucose (see Figure 9.9). Notice in Figure 10.18 that for every three molecules of CO 2 that enter the cycle, there are six molecules of G3P formed. But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate. The cycle began with 15 carbons' worth of carbohydrate in the form of three molecules of the five-carbon sugar RuBP. Now there are 18 carbons' worth of carbohydrate in the form of six molecules of G3P. One molecule exits the cycle to be used by the plant cell, but the other five molecules must be recycled to regenerate the three molecules of RuB? Phase 3: Regeneration of the CO2 acceptor (RuBP). In a complex series of reactions. the carbon skeletons of fke molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. To accomplish this, the cycle spends three more molecules of ATP. The RuBP is now prepared to receive CO 2 again, and the cycle continues. For the net synthesis of one G3P molecule, the Calvin cycle consumes a total of nine molecules of ATP and six molecules
Input
3g (Entering one CO at a time)
,
Phase 1; Carbon fiKation
Rubisco 3
'1 'l..-J
Short-lived........ intermediate
6Q-Q-Q-®
3~ Ribulose bisphosphate (RuBP)
3·Phosphoglycerate
~
~--6~
\~-. 6AOP
3ADP
~
__
Caillin Cyde
3$----
6~ 1.3-Bisphosphoglycerate
I~--- 61NADPHI
Phase 3;
--·6NAOP+
Regeneration of
the CO 2 acceptor (RuBP)
--~6®,
sQ-Q-Q-® G3P
6Q-Q-Q-® Glyceraldehyde·3·phosphate (G3P)
lQ-O-O-® ... Figure 10.18 The Calvin cycle. This diagram trilcks carbon atoms (gray balls) through the cycle. The three phases of the cycle correspond to the phases discussed in the text. For every three molecules of COl that enter the cycle. the net output is one molecule of glyceraldehyde-3-phosphale (G3P), a three-carbon sugar. The light reactions sustain the Calvin cycle by regenerating AlP and NADPH. ••I/W"I Redraw this l¥/e using numerals to Indicate the numbers of carbons instead of gray balls, multiPlYing at each step to ensure that you have iKCoonted for all carbons. In W'har forms do the carbon aromsenrer
and leave the cycle)
of NADPH. The light reactions regenerate the ATP and NADPH. TheG3P spun off from the Calvin cycle becomes the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates. Neither the light reactions nor the Calvin cycle alone can make sugar from CO 2 , Photosynthesis is an emergent property of the intact chloroplast, which integrates the two stages of photosynthesis.
G3P a sugar Output
CONCEPT
Glucose and other organic compounds
CHECK
10.3
1. To synthesize one glucose molecule, the Calvin cycle uses molecules of COo molecules of ATp, and moleculesofNADPH. 2. Explain why the large numbers of ATP and NADPH molecules used during the Calvin cycle are consistent with the high value of glucose as an energy source. UI A Explain why a poison that inhibits an 3. enzyme of the Calvin cycle will also inhibit the light reactions.
-·,1m
For suggested answers. see AppendiK A.
CHAPTH TEN
Photosynthesis
199
rZ~~:~:~~O~:chanisms of
carbon fixation have evolved in hot, arid climates
Ever since plants first moved onto land about 475 million years ago, they have been adapting to the problems of terrestrial life, particularly the problem of dehydration. In Chapters 29 and 36, we will consider anatomical adaptations that help plants conserve water. Here we are concerned with metabolic adaptations. The solutions often involve tradeoffs. An important example is the compromise between photosynthesis and the prevention of excessive water loss from the plant. The CO 2 required for photosynthesis enters a leaf via stomata, the pores through the leaf surface (see Figure 10.3). However, stomata are also the main avenues of transpiration, the evaporative loss of water from leaves. On a hot, dry day, most plants close their stomata, a response that conserves water. This response also reduces photosynthetic yield by limiting access to CO 2 , With stomata even partially closed, CO 2 concentrations begin to decrease in the air spaces within the leaf, and the concentration of O 2 released from the light reactions begins to increase. These conditions within the leaf favor an apparently wasteful process called photorespiration.
Photorespiration: An Evolutionary Relic? In most plants, initial fixation of carbon occurs via rubisco, the Calvin cycle enzyme that adds CO 2 to ribulose bisphosphate. Such plants are called C3 plants because the first organic product of carbon fixation is a three-carbon compound, 3-phosphoglycerate (see Figure 10.18). Rice, wheat, and soybeans are C3 plants that are important in agriculture. When their stomata partially close on hot, dry days, C3 plants produce less sugar because the declining level of CO 2 in the leaf starves the Calvin cycle. In addition, rubisco can bind O 2 in place ofCO2 , As CO2 becomes scarce within the air spaces of the leaf, rubisco adds O 2 to the Calvin cycle instead of CO2 , The product splits, and a two~carbon compound leaves the chloroplast. Peroxisomes and mitochondria rearrange and split this compound, releasing CO 2 , The process is called photorcspiration because it occurs in the light (photo) and consumes O2 while producing CO 2 (respiration). However, unlike normal cellular respiration, photorespiration generates no ATP; in fact, photorespiration consumes ATP. And unlike photosynthesis, photorespiration produces no sugar. In fact, photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle and releasing CO 2 that would otherwise be fixed. How can we explain the existence of a metabolic process that seems to be counterproductive for the plant? According 200
UNIT TWO
The Cell
to one hypothesis, photorespiration is evolutionary baggagea metabolic relic from a much earlier time when the atmosphere had less O 2 and more CO 2 than it does today. In the ancient atmosphere that prevailed when rubisco first evolved, the inability of the enzyme's active site to exclude O2 would have made little difference. The hypothesis suggests that modern rubisco retains some of its chance affinity for 02> which is now so concentrated in the atmosphere that a certain amount of photorespiration is inevitable. We now know that, at least in some cases, photorespiration plays a protective role in plants. Plants that are impaired in their ability to carry out photorespiration (due to defective genes) are more susceptible to damage induced by excess light. Researchers consider this clear evidence that photorespiration acts to neutralize the otherwise damaging products ofthe light reac~ tions, which build up when a low CO 2 concentration limits the progress ofthe Calvin cycle. Whether there are other benefits of photorespiration is still unknown. In many types of plantsincluding a significant number of crop plants-photorespiration drains away as much as 50% of the carbon fixed by the Calvin cycle. As heterotrophs that depend on carbon fixation in chloroplasts for our food, we naturally view photorespiration as wasteful. Indeed, ifphotorespiration could be reduced in certain plant species without otherwise affecting photosynthetic productivity, crop yields and food supplies might increase. In some plant species, alternate modes of carbon fixation have evolved that minimize photorespiration and optimize the Calvin cycle-even in hot, arid climates. The two most im~ portant of these photosynthetic adaptations are C4 photosynthesis and CAM.
(4
Plants
The C4 plants are so named because they preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product. Several thousand species in at least 19 plant families use the C4 pathway. Among the C4 plants important to agriculture are sugarcane and corn, members of the grass family. A unique leaf anatomy is correlated with the mechanism of C4 photosynthesis (Figure 10.19; compare with Figure 10.3). In C4 plants, there are two distinct types of photosynthetic cells: bundle~sheath cells and mesophyll cells. Bundle-sheath cells are arranged into tightly packed sheaths around the veins of the leaf. Bern'een the bundle sheath and the leaf surface are the more loosely arranged mesophyll cells. The Calvin cycle is confined to the chloroplasts of the bundle-sheath cells. However, the cycle is preceded by incorporation of CO 2 into organic compounds in the mesophyll cells (see the numbered steps in Figure 10.19). 0 The first step is carried out by an enzyme present only in mesophyll cells called PEP carboxylase. This enzyme adds CO2 to phosphoenolpyruvate (PEP), forming the four-carbon product oxaloacetate. PEP carboxylase
Mesophyll cell PEP carboxylase
Mesophyll Cell\
Photosynthetic cells of (4 plant Bundle{ sheath leaf cell
~'~~':."'~"i.-:'.
The C4 pathway
~'
.
Oln mesophyll cells. the enzyme PEP carboxylase adds carbon dioxide to PEP.
Vein -{' (vascular tissue)
(lJIrfi1J:;""
ecompound Afour-carbon
c. leaf anatomy
conveys the atoms of the CO 2 into a bundle-sheath cell via plasmodesmata.
Stoma
ocells,In CObundle-sheath is 2
released and enters the Calvin cycle.
... Figure 10.19 4 leaf anatomy and the 4 pathway. The structure and biochemical functions of the lea~es of C4 plants are an e~olutionary adaptation to hot, dry climates. This adaptation maintains a COl concentration in the bundle sheath that fa~ors photosynthesis o~er photorespiration
has a much higher affinity for CO 2 than does rubisco and no affinity for O 2, Therefore, PEP carboxylase can fix carbon efficiently when rubisco cannot-that is, when it is hot and dry and stomata are partially closed, causing CO 2 concentration in the leaf to fall and O 2 concentration to rise. f.) After the Colplant fixes carbon from COb the mesophyll cells export their four-carbon products (malate in the example shown in figure 10.19) to bundle-sheath cells through plasmodesmata Within the bundle-sheath cells, the (see Figure 6.31). four-carbon compounds release CO 2, which is reassimilated into organic material by rubisco and the Calvin cycle. The same reaction regenerates pyruvate, which is transported to mesophyll cells. There, ATP is used to convert pyruvate to PEP, allowing the reaction cycle to continue; this ATP can be thought of as the "price" of concentrating CO 2 in the bundlesheath cells. To generate this extra ATP, bundle-sheath cells carry out cyclic electron flow, the process described earlier in this chapter (see Figure 10.15). In fact, these cells contain PS I but no PS II, so cyclic electron flow is their only photosynthetic mode of generating ATP. In effect, the mesophyll cells ofa C4 plant pump CO 2 into the bundle sheath, keeping the CO2 concentration in the bundlesheath cells high enough for rubisco to bind carbon dioxide rather than oxygen. The cyclic series of reactions involving PEP carboxylase and the regeneration of PEP can be thought of as a COrconcentrating pump that is powered by ATP. In
e
this way, C 4 photosynthesis minimizes photorespiration and enhances sugar production. This adaptation is especially advantageous in hot regions with intense sunlight, where stomata partially close during the day, and it is in such environments that C4 plants evolved and thrive today.
CAM Plants A second photosynthetic adaptation to arid conditions has evolved in many succulent (water-storing) plants, numerous cacti, pineapples, and representatives of several other plant families. These plants open their stomata during the night and close them during the day, just the reverse of how other plants behave. Closing stomata during the day helps desert plants conserve water, but it also prevents CO 2 from entering the leaves. During the night, when their stomata are open, these plants take up CO2 and incorporate it into a variety of organic acids. This mode of carbon fixation is called crassulacean acid metabolism, or CAM, after the plant family Crassulaceae, the succulents in which the process was first discovered. The mesophyll cells of CAM plants store the organic acids they make during the night in their vacuoles until morning, when the stomata close. During the day, when the light reactions can supply ATP and NADPH for the Calvin cycle, CO 2 is released from the organic acids made the night before to become incorporated into sugar in the chloroplasts.
CHAPTH TEN
Photosynthesis
201
,. Figure 10.20 C. and CAM photosynthesis compared. Both adaptations are characterized by 0 preliminary incorporation of COl into organic acids. followed by transfer of COl to the Calvin cycle. The C4 and CAM pathways are two evolutionary solutions to the problem of maintaining photosynthesis with stomata partially or completely closed on hot. dry days,
e
Sugarcane
Pineapple
CAM
C,
oce
CO,
Mesophyll cell
Bundlesheath cell
o into COl incorporated four-carbon organic acids (carbon fixation)
CO Calvin Cycle
CO
8
Sugar
CONCEPT
CHECI(
10.4
1. Explain why photorespiration lowers photosynthetic output for plants. 2. The presence of only PS I, not PS II, in the bundlesheath cells of C4 plants has an effect on O 2 concentration. What is that effect, and how might that benefit the plant? 3, How would you expect the relative abundance of C:3 versus C 4 and CAM species to change in a geographic region whose climate becomes much hotter and drier?
_lm'路',14
For suggested answers. see Appendix A,
202
UNIT TWO
The Cell
Night
D',
Organic acids release COl to Calvin cycle Sugar
(a) Spatial separation of steps. In C4 plants. carbon fixation and the Calvin cycle occur in different types of cells,
Notice in Figure 10.20 that the CAM pathway is similar to the Ct pathway in that carbon dioxide is first incorporated into organic intermediates before it enters the Calvin cycle. The difference is that in C4 plants, the initial steps ofcarbon fixation are separated structurally from the Calvin cycle, whereas in CAM plants, the two steps occur at separate times but within the same cell. (Keep in mind that CAM, C"" and C3 plants all eventually use the Calvin cycle to make sugar from carbon dioxide.)
oco
CO,
(b) Temporal separation of steps. In CAM plants. carbon fixation and the Calvin cycle occur in the same cells at differer'lt times.
The Importance of Photosynthesis: A Review In this chapter, we have followed photosynthesis from pho路 tons to food. The light reactions capture solar energy and use it to make ATP and transfer electrons from water to NADP+, forming NADPH. The Calvin cycle uses the ATP and NADPH to produce sugar from carbon dioxide. The energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds. See Figure 10.21 for a review of the entire process. What are the fates of photosynthetic products? The sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons for the synthesis of all the major organic molecules of plant cells. About 50% of the organic material made by photosynthesis is consumed as fuel for cellular respiration in the mitochondria of the plant cells. Sometimes there is a loss ofphotosynthetic products to photo路 respiration. Technically, green cells are the only autotrophic parts of the plant. The rest of the plant depends on organic molecules exported from leaves via veins. In most plants, carbohydrate is transported out of the leaves in the form of
•
•
light
Light Reactions: Photosystem 11 E\!Ctron transport chain " Photosystem I . ~ Electron transport chain '- ~
'I
3-Phosphoglycerate
C""") Cycle
AlP NADPH
G3P
I.,../"
Starch (storage)
Chloroplast
o Light Reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of AlP and NADPH • Split H20 and release 02 to the atmosphere
Sucrose (export) Calvin Cycle Reactions: • Take place in the stroma • Use ATP and NADPH to convert CO 2 to the sugar G3P • Return ADp, inorganic phosphate, and NADP" to the light reactions
... Figure 10.21 A review of photosynthesis. ThiS diagram outlines the main reactants and products of the light reactions and the Calvin cycle as they occur in the chloroplasts of plant cells. The entire ordered operation depends on the structural integrity of the chloroplast and its membranes. Enzymes in the chloroplast and cytosol convert glyceraldehyde-3-phosphate (G3P), the direct product of the Calvin cycle, to many other organic compounds.
sucrose, a disaccharide. After arriving at nonphotosynthetic cells, the sucrose provides raw material for cellular respiration and a multitude of anabolic pathways that synthesize proteins, lipids, and other products. A considerable amount of sugar in the form ofglucose is linked together to make the polysaccharide cellulose, especially in plant cells that are still growing and maturing. Cellulose, the main ingredient of cell walls, is the most abundant organic molecule in the plantand probably on the surface of the planet. Most plants manage to make more organic material each day than they need to use as respiratory fuel and precursors for biosynthesis. They stockpile the extra sugar by synthesizing starch, storing some in the chloroplasts themselves and some in storage cells of roots, tubers, seeds, and fruits. In accounting for the consumption of the food mol-
ecules produced by photosynthesis, let's not forget that most plants lose leaves, roots, stems, fruits, and sometimes their entire bodies to heterotrophs, including humans. On a global scale, photosynthesis is the process responsi· ble for the presence of oxygen in our atmosphere. Furthermore, in terms of food production, the collective productivity ofthe minuscule chloroplasts is prodigious: Photosynthesis makes an estimated 160 billion metric tons of carbohydrate per year (a metric ton is 1,000 kg, about 1.1 tons). That's organic matter equivalent in mass to a stack of about 60 trillion copies of this textbook-I7 stacks of books reaching from Earth to the sun! No other chemical process on the planet can match the output of photosynthesis. And no process is more important than photosynthesis to the welfare of life on Earth. CHAPTH TEN
Photosynthesis
203
a -MH'·.
.. ter;lf 1'1
Go to the Study Area at www.masteringbio,com for BioFlix
3-D Animations, MP3 Tutors, Videos, Practice Tests, an eBook, and more.
eVlew
.. Linear Electron Flow The flow of electrons during the light reactions produces NADPH, ATP, and oxygen:
SUMMARY OF KEY CONCEPTS
_ •.lllill-10.1
• .:•
Photosynthesis converts light energy to the chemical energy of food (pp. 18&-189) .. Chloroplasts: The Sites of Photosynthesis in Plants In au-
"""PH
r......
totrophic eukaryotes, photosynthesis occurs in chloroplasts, organelles containing th}~akoids. Stacks of thylakoids form grana.
Photo,ystem I
.. Tracking Atoms Through Photosynthesis: Scie1ltific [11quiry Photosynthesis is summarized as 6CG.! + 12 H~O + Light energy ) 4HI10S + 6 O:! + 6 HzO Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules. Photosynthesis is a redox process: H1 0 is oxidized, C01 is reduced.
.. The Two Stages of Photosynthesis: A Preview The light reactions in the thylakoid membranes split water, releasing O2, producing ATp, and forming NADPH. The Calvin cycle in the stroma forms sugar from CO:z, using ATP for energy and NADPH for reducing power. RioFlix J.I) Animation Photosynthesis MPJ Tutor Photosynthesis Activity The Site, ofPhotosynthe,is Activity Overview of Photosynthesis
.','1'''''-10.2 The light reactions convert solar energy to the chemical energy of AlP and NADPH (pp_ 190-198)
.. Cyclic Electron Flow Cyclic electron flow employs only photosystem I, producing ATP but no NADPH or O 2, .. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria In both organelles, redox reactions of electron transport chains generate an H+ gradient across a membrane. ATP synthase uses this proton-motive force to make ATP. Acchity Light Energy and Pigments In'·estigation How Does PapcrChromatography Separate Plant Pigments? Activity The Light Reactions
."'i'ill-10.3 The Calvin cycle uses AlP and NADPH to convert CO 2 to sugar (pp. 198-199) .. The Calvin cycle occurs in the stroma, using electrons from NADPH and energy from ATP. One molecule ofG3P exits the cycle per three C01 molecules fixed and is converted to glucose and other organic molecules.
.. The Nature of Sunlight Ught is a form of electromagnetic energy. The colors we see as visible light include those wavelengths that drive photosynthesis. Carbon fixation
.. Photosynthetic Pigments: The Light Receptors A pigment absorbs visible light of specific wavelengths. Chlorophyll a is the main photosynthetic pigment in plants. Other accessory pigments absorb different wavelengths oflight and pass the energy on to chlorophyll a. .. Excitation of Chlorophyll by light A pigment goes from a ground state to an excited state when a photon boosts one of its electrons to a higher-energy orbital. This excited state is unstable. Electrons from isolated pigments tend to fall back to the ground state, giving off heat and/or light. .. A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes A photosystem is com· posed of a reaction-center complex surrounded by light. harvesting complexes that funnel the energy of photons to the reaction·center complex. When a special pair of reaction-center chlorophyll a molecules absorbs energy, one of its electrons is boosted to a higher energy level and tmnsferred to the primary electron acceptor. Photosystem II contains P680 chlorophyll a molecules in the reaction·center complex; photosystem I contains P700 molecules.
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UNIT TWO
The Cell
CaMn Cycle Re-generation 01 COl il(ceptor
5dC ReductIOn
1 G3P(KI
Activity The Calvin Cycle lnnsligation How Is the Rate of Photosynthesis Measured? Biology Labs On' Line LealLlb
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1'-10.4
6. \X'hich of the following statements is a correct distinction between autotrophs and heterotrophs? a. Only heterotrophs require chemical compounds from the environment. b. Cellular respiration is unique to heterotrophs. c. Only heterotrophs have mitochondria. d. Autotrophs, but not heterotrophs, can nourish themselves beginning with CO 2 and other nutrients that are inorganic. e. Only heterotrophs require oxygen. 7. Which of the following does not occur during the Calvin cycle? a. carbon fixation b. oxidation ofNADPH c. release of oxygen d. regeneration of the CO 2 acceptor e. consumption of ATP
-51401"·
Acthity Photo'ynthe,is in Dry Oimates
... The Importance of Photosynthesis: A Review Organic compounds produced by photosynthesis provide the energy and building material for ecosystems. TESTING YOUR KNOWLEDGE
SELF-QUIZ 1. The light reactions of photosynthesis supply the Calvin cycle with a. light energy. d. ATP and NADPH. b. CO 2 and ATP. e. sugar and O2, c. H20 and NADPH. 2. Which of the following sequences correctly represents the flow ofelectrons during photosynthesis? a. NADPH ---> O2 -. CO 2 b. H20 ---> NADPH ---> Calvin cycle Co NADPH ---> chlorophyll---> Calvin cycle d. H20 ---> photosystem I ---> photosystem 11 e. NADPH ---> electron transport chain ---> O2 3. In mechanism, photophosphorylation is most similar to a. substrate-level phosphorylation in glycolysis. b. oxidative phosphorylation in cellular respiration. c. the Calvin cycle. d. carbon fixation. e. reductionofNADP+. 4. How is photosynthesis similar in C4 plants and CAM plants? a. In both cases, only photosystem J is used. b. Both types of plants make sugar without the Calvin cycle. c. In both cases, rubisco is not used to fix carbon initially. d. Both types of plants make most of their sugar in the dark. e. In both cases. thylakoids are not involved in photosynthesis. 5. Which process is most directly driven by light energy? a. creation of a pH gradient by pumping protons across the thylakoid membrane b. carbon fixation in the stroma Co reduction of NADP- molecules d. removal of electrons from chlorophyll molecules e. ATP synthesis
For Self· Quiz answers, see Appendix A.
-51401',. Visit the Study Area at www.masteringbio.com lor a Practice Test
EVOLUTION CONNECTION 8. Photorespiration can decrease soybeans' photosynthetic output by about 50%. Would you expect this figure to be higher or lower in wild relatives of soybeans? Why?
SCIENTIFIC INQUIRY 9. •• I&W"I The following diagram represents an experiment with isolated chloroplasts. The chloroplasts were first made acidic by soaking them in a solution at pH 4. After the thylakoid space reached pH 4, the chloroplasts were transferred to a basic solution at pH 8. The chloroplasts then made ATP in the dark.
(@g)-----:::::'T-,..;,..-@>-----:::::~ pH?
pH4
pH4
@>
pH 8
Draw an enlargement of part of the thytakoid membrane in the beaker with the solution at pH 8. Draw ATP synthase. Label the areas of high H+ concentration and low HT concentration. Show the direction protons flow through the enzyme, and show the reaction where ATP is synthesized. Would ATP end up in the thylakoid or outside of it? Explain why the chloroplasts in the experiment were able to make ATP in the dark.
SCIENCE, TECHNOLOGY, AND SOCIETY 10. Scientific evidence indiCltes that the CO 2 added to the air by the burning ofwood and fossil fuels is contributing to "glohal warming;' a rise in global temperature. Tropical min forests are estimated to be responsible for more than 20% ofglobal photosynthesis, yet their consumption of large anmunts of C~ is thought to make little or no net contribution to reduction ofglohal warming. \X!hy might this be? (Hint: \X!hat happens to the food produced bya rain forest tree when it is eaten by animals or the tree dies?)
CHAPTH TEN
Photosynthesis
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